Anisotropic titanate etch

ABSTRACT

A titanate substrate (e.g. lead zirconate titanate 34) is immersed in a liquid ambient (e.g. 12 molar concentration hydrochloric acid 30) and illuminated with radiation (e.g. collimated visible/ultraviolet radiation 24) produced by a radiation source (e.g. a 200 Watt mercury xenon arc lamp 20). A window 26 which is substantially transparent to the collimated radiation 24 allows the radiated energy to reach the titanate substrate 34. An etch mask 32 may be positioned between the radiation source 20 and the substrate 34. The titanate substrate 34 and liquid ambient 30 are maintained at a nominal temperature (e.g. 25° C.). Without illumination, the titanate is not appreciably etched by the liquid ambient. Upon illumination the etch rate is substantially increased.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following related applications were filed concurrently with the instant application:

    ______________________________________                                                        First Named                                                     Title          Inventor    Ser. No./Status                                     ______________________________________                                         Anisotropic Metal Oxide                                                                       Douglas     07/871,862; pending                                 Etch                                                                           Anisotropic Barium                                                                            Douglas     07/870,458; allowed                                 Strontium Titanate Etch                                                        Anisotropic Niobium                                                                           Douglas     07/872,701; allowed                                 Pentoxide Etch                                                                 Anisotropic Tantalum                                                                          Douglas     07/872,702; allowed                                 Pentoxide Etch                                                                 ______________________________________                                    

FIELD OF THE INVENTION

This invention generally relates to methods of anisotropically etching titanate materials.

BACKGROUND OF THE INVENTION

Without limiting the scope of the invention, its background is described in connection with current methods of etching titanates, as an example.

Titanate materials (e.g. lead zirconate titanate, PbTiZrO₃, hereafter abbreviated PZT), because of their electrical and mechanical properties, have found many uses in the field of electronics. The very high dielectric constants exhibited by many titanate compounds make them useful as the dielectric material in capacitors, especially in the miniature capacitors which are built into many integrated circuits (e.g dynamic RAMs). Many titanate materials also have a positive temperature coefficient of electrical resistance, which allows devices to be made which protect electrical motors from damage due to over-current conditions. The piezoelectrical properties of these materials are widely used, as in the precise control of the length of laser cavities, for example. Microminiature structures which incorporate titanate materials are being used to sense infrared radiation, obviating the need for bandgap detector materials which require cryogenic cooling to sense the infrared.

Titanate materials are often etched during the fabrication of the electrical devices which exploit their beneficial properties. A method used to etch titanates should generally do so without introducing damage which would unacceptably change the properties of the material on which the function of the eventual devices depends. In addition, an anisotropic etch method is usually desired so that detail in the etch mask pattern is preserved. Contamination of the titanate material (and/or nearby materials) by the etch method usually cannot be tolerated.

Heretofore, in this field, titanates have been etched by isotropic wet etching, ion-milling, plasma etching or laser scribing. Laser scribing is a method wherein selected portions of the material are damaged and weakened by exposure to intense laser radiation and then removed.

SUMMARY OF THE INVENTION

It has been discovered that current methods of etching titanate materials can produce unacceptable results. Isotropic wet etching can undercut the etch mask (generally by the same distance as the etch depth), leading to features in the underlying titanate material which are not true to the etch mask pattern. Ion-milling can generate unacceptable defects in the titanate material, including changes in its electrical, chemical, mechanical, optical, and/or magnetic properties. Plasma etching can cause damage similar to that caused by ion-milling, and in addition the plasma constituent elements can contaminate substrate materials and are often difficult to remove. Etching by laser scribing can result in no undercut of the material, but the laser radiation can cause damage to the titanate material which must be repaired by a high-temperature (approximately 1400° C.) anneal. This anneal may produce undesirable oxidation states in the titanate material, reducing its beneficial properties. High temperature anneals, in addition, can cause changes and damage to other materials and structures present in the substrate.

Generally, and in one form of the invention, etching of titanate materials is accomplished by immersing them in a liquid ambient and then exposing them to electromagnetic radiation (e.g. ultraviolet light), causing illuminated portions of the substrate to be etched and unilluminated portions to remain substantially unetched. The process presented is therefore an anisotropic liquid phase photochemical etch. Preferably, PZT is etched by immersing it in hydrochloric acid and illuminating it with visible and ultraviolet radiation provided by a mercury xenon arc lamp. Hydrofluoric acid may also be used. An etch mask is used to define the pattern of illumination at the surface and thereby define the etch pattern. The highly directional nature of light makes this an anisotropic etch method.

This is apparently the first titanate etch method to make use of electronic excitation caused by photo-irradiation in a reaction between a titanate and a non-alkaline liquid ambient (e.g. hydrochloric and/or hydrofluoric acid). An advantage of the invention is that it anisotropic (i.e. does not cause substantial undercut). Those regions under the mask remain in shadow and are not appreciably etched. In addition, it has been found that this method does not cause unacceptable defects in the titanate material. Contamination from plasma species is prevented. Generally, no high temperature anneal is required.

BRIEF DESCRIPTION OF THE DRAWING

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to the detailed description which follows, read in conjunction with the accompanying drawing.

In the drawing:

FIG. 1 is a representation of the apparatus and materials used to anisotropically etch titanate materials.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

In the preferred embodiment of this invention and with reference to FIG. 1, a pattern is etched into a substrate 34 of lead zirconate titanate (PbTiZrO₃) which is immersed in 12 molar concentration hydrochloric acid (HCl) 30 and illuminated with substantially collimated visible/ultraviolet radiation 24 propagating substantially orthogonal to the substrate surface 34 produced by a 200 Watt mercury xenon arc lamp 20. Radiation 21 from the radiation source 20 is collimated by collimating optics 22 and the resulting collimated radiation 24 is directed at the substrate 34. A window 26 which is a portion of the reaction vessel 28 and which is substantially transparent to the collimated radiation allows the radiated energy to reach the PZT substrate 34. An etch mask 32 defines the etched pattern by blocking the radiation at portions of the substrate. The PZT substrate 34 and acid solution 30 are nominally at a temperature of 25° C. At such a temperature and acid concentration and without illumination, PZT is not appreciably etched by HCl. Upon illumination by visible/ultraviolet radiation produced by the lamp, however, the etch rate is increased. It is believed, in part because of the relatively low power of the light source, that the etching reaction is accelerated in the illuminated areas because of electronic excitation due to photo-irradiation rather than from thermal effects. As used herein, the term "radiation" means radiation at levels above background and this means, for example, illumination at levels substantially greater than room lighting.

The etch mask 32 between the radiation source 20 and the PZT substrate 34 is preferably located close to or in contact with the surface to be etched. Such an etch mask 32 may be deposited onto the substrate surface. The etch mask 32 is preferably made of silicon oxide. In general, any substance which is sufficiently opaque to visible and ultraviolet light such as that produced by the mercury xenon lamp and which is not substantially penetrated or etched by the liquid ambient may be used, such as silicon oxide or silicon nitride. Those areas of the substrate not covered by the mask material will be subject to etching by the acid solution.

In alternate embodiments, the liquid ambient may be from the class of solutions that etch the material without light irradiation. In this case, the radiation accelerates the etch rate on illuminated portions of the substrate, resulting in a less isotropic etch. Still other alternate embodiments include liquid ambients containing salts and liquid ambients with pH values less than or equal to seven (i.e. acids and neutral solutions).

In an additional aspect to this method, sidewall profile control can be enhanced by introducing a passivating agent into the liquid ambient 30 that does not react with the material being etched to form a soluble product or only slightly soluble product. The passivating agent "poisons" the sidewalls with respect to attack by other etch reagents on the sidewall, but does not stop etching normal to the substrate because the irradiation detaches the passivating agent from the surface. An example of a passivating agent in an etch of PZT is phosphoric acid.

In another alternate embodiment of this invention, a pattern may be etched into the surface of a titanate material by projecting a patterned light onto the substrate (e.g. by conventional photo-lithographic techniques).

In yet other alternate embodiments of this invention, the liquid ambient may be made to flow with respect to the substrate. The flow rate of the liquid ambient may be varied. The solution temperature can be varied to achieve different etch rates and etch anisotropy. The photon flux may be varied to impact etch directionality and etch rates. The radiation wavelength can be adjusted to achieve different etch directionality and etch rates. The direction of propagation of the radiation need not be normal to the surface. The etch solution may be a mixture of solutions (e.g. one or more passivating agents to enhance anisotropy and one or more reagents to photochemically etch the material).

The sole Table, below, provides an overview of some embodiments and the drawing.

                  TABLE                                                            ______________________________________                                         Draw-                                                                          ing                                                                            Ele-  Generic    Preferred or                                                  ment  Term       Specific Term                                                                              Alternate Terms                                   ______________________________________                                         20    Radiation  200 Watt                                                            Source     mercury/xenon                                                                  arc lamp                                                      21    Radiation  Visibile/UV Radiation which will                                               light       penetrate the liquid                                                           ambient                                           22    Collimating                                                                    Optics                                                                   24    Collimated Visible/UV                                                          Radiation  light propaga-                                                                 ting normal to                                                                 substrate                                                                      surface                                                       26    Transparent                                                                    Window                                                                   28    Reaction                                                                       Vessel                                                                   30    Liquid     12 Molar    Other etch constituents                                 Ambient    Hydrochloric                                                                               such as hydrofluoric acid                                          acid        (HF);                                                                          Passivating agents such                                                        as phosphoric acid.                                                            Mixtures of both.                                 32    Etch Mask  Silicon oxide                                                                              silicon nitride                                                                Noble metals such as                                                           platinum                                          34    Titanate   Lead zirconate                                                                             Other titanate com-                                     Substrate  titanate,   pounds such as barium                                              PbTiZrO.sub.3                                                                              strontium titanate.                                                            Transition titanates                                                           such as LiNbO.sub.3                               ______________________________________                                    

A few embodiments have been described in detail hereinabove. It is to be understood that the scope of the invention also comprehends embodiments different form those described, yet within the scope of the claims.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. For example, although alkaline solutions are not preferred, they may be the appropriate liquid ambient in some embodiments of this invention. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

What is claimed is:
 1. A method to anisotropically etch a titanate material comprising the steps of(a) submersing a surface of said titanate material in a non-alkaline liquid ambient; and (b) illuminating portions of said surface with radiation, thereby etching said illuminated portions of said surface at a substantially greater rate than the unilluminated portions of said surface.
 2. The method according to claim 1, wherein said illuminated portions of said surface are at a temperature substantially the same as the temperature of the unilluminated portions of said surface.
 3. The method according to claim 1, wherein said liquid ambient is comprised of hydrochloric acid or hydrofluoric acid or a combination thereof.
 4. The method to claim 1, wherein said radiation is produced by a mercury xenon arc lamp.
 5. The method according to claim 1, wherein said titanate material is lead zirconate titanate.
 6. The method according to claim 1, wherein said titanate material is barium strontium titanate.
 7. The method according to claim 1, wherein the direction of propagation of said radiation is substantially orthogonal to said surface.
 8. The method according to claim 1, wherein an etch mask is interposed between the source of said radiation and said surface, whereby portions of said surface are not illuminated.
 9. The method according to claim 8, wherein said etch mask is silicon oxide.
 10. The method according to claim 8, wherein said etch mask is silicon nitride.
 11. The method according to claim 8, wherein said etch mask is a noble metal.
 12. The method according to claim 11, wherein said noble metal is platinum.
 13. The method according to claim 1, wherein said liquid ambient is acidic.
 14. The method according to claim 1, wherein said liquid ambient is comprised of a salt solution.
 15. A method to anisotropically etch lead zirconate titanate comprising the steps of(a) immersing a surface of said lead zirconate titanate in 12 molar hydrochloric acid; and (b) illuminating portions of said surface with radiation produced by a mercury xenon arc lamp, thereby etching said illuminated portions of said surface at a substantially greater rate than the unilluminated portions of said surface.
 16. A method of anisotropically etch a titanate material comprising the steps of(a) submersing a surface of said titanate material in a liquid ambient; and (b) illuminating portions of said surface with radiation,thereby etching said illuminated portions of said surface at a substantially greater rate than the unilluminated portions of said surface. 